Using bulk phyllosilicates and clays as fillers in industrial products, including plastics and paper products, is well known. Typically, phyllosilicates and clays have been used in materials as fillers because the cost of the bulk phyllosilicate or clay is much less than the material to which the phyllosilicate is added. Conventional techniques and equipment allow phyllosilicates to be dispersed in roughly micrometer sized particles throughout a substance. Although using bulk phyllosilicates as fillers has decreased the cost of many finished products, the use of phyllosilicates in these products has, at times, had a detrimental effect on the physical characteristics of the completed product, including excessive weight gain and decreased physical strength.
There has been a great deal of interest in the last five years to incorporate fully exfoliated smectite clays, primarily montmorillonite with its extremely high aspect ratio, into a variety of polymers, particularly nylon, polyolefins and polyethyleneterephthalate (PET). The approach taken by most research groups is based on the technology utilized for the last forty years to make organoclays as rheological control agents in paints, inks, greases, and the like. This approach utilizes quaternary amine-based surfactants to render the surface of the clay compatible with the polymer matrix. The concept being that the clay surface is very hydrophilic and the quaternary amines render the surface hydrophobic and thus, more compatible with the polymers which themselves are very hydrophobic. This approach, however, has not been very successful in fully dispersing the clay platelets through exfoliation throughout the polymer matrix. Another problem with this class of surfactants is that the amines tend to decompose around or below the temperatures needed to extrude these polymers. Thus, polymer nanocomposites have proven difficult to manufacture. Other attempts to overcome this inherent incompatibility between the phyllosilicate and the polymer include chemically treating the phyllosilicate to make the phyllosilicate more polymer compatible. Such treatment is followed by melt compounding and intercalation of the clay with monomers which are then polymerized into the polymer matrix through condensation or free radical polymerization. However, these processes are costly, time consuming, and have only been successful for combining a limited range of clays with specific polymers.
Current technology, based on ammonium ion surfactants, has been surprisingly unsuccessful at incorporating phyllosilicates into polymers to produce nanocompositions. Organophyllosilicates tend to be either completely hydrophilic or completely hydrophobic with no ability to shift their surface wetting characteristics in situ. A shift from hydrophilic to hydrophobic surface wetting is an especially valuable characteristic in the production of films using water-based compositions which impart water and chemical resistance to the substrate when dry. In addition, the onium ion surfactants that have been used extensively in the past to make organophyllosilicates rely on the ion exchange reaction of a single functional group in the surfactant molecule. This means that the individual surfactant molecules are relatively weakly attached to the clay surface and can be stripped off in the high temperature/high shear environment of the extruders and mixers that have been used in blending and exfoliating the organoclays with the polymer melt.
Thus there is a need for more robust surface treatment technology to enable dispersion of natural phyllosilicates and hydrophilic organophyllosilicates into various polymer systems to form nanocomposites.